Solid particle erosion of turbine control stages severely impacts the performance of a turbine. To restore performance, procedures have been developed for replacing the eroded steam path of a nozzle box with a modern-profile diffusion-coated erosion resistant steam path. The cycle time needed for replacement, however, requires that the utility have a spare nozzle box, or enough similar nozzle boxes, to justify the purchase of a spare. Without a spare, the savings due to continuous improved performance does not justify the cost of the extended outage time needed to replace the steam path plus the cost of the steam path replacement itself.
In order to provide modern erosion resistant steam paths to "one of a kind" nozzle box owners, the steam path replacement cycle had to be reduced to fit into a normal planned outage.
By way of a more detailed background, and with reference to FIG. 1, the control or first stage 10 of a conventional turbine includes a nozzle box 12 surrounding a rotor 14, the turbine control stage represented by a single bucket 16. The nozzle box 12 in accordance with conventional practice includes, generally, a torus portion 18, a bridge ring assembly 20 and a partition ring assembly 22. In conventional turbines of this type, steam is fed into the torus portion 18 of the nozzle box 12, and directed axially between the outer bridge ring 24 and the inner bridge ring 26, and between a plurality of circumferentially spaced bridge elements 28 (cross-sectional shape illustrated in FIG. 1a) which connect the rings 24 and 26. The steam then flows through the steam path assembly 22, towards the buckets 16. The partition ring assembly in prior turbine constructions consists of radially inner and outer bands 30 and 32 (each formed in 180.degree. segments which, when the turbine is fully assembled, form 360.degree. rings), respectively, which held between them a large number (for example, 100) of vane-shaped partition elements 34 (cross-sectional shape illustrated in FIG. 1b) which serve to direct the steam at a desired angle to the bucket blades. The steampath assembly 22 is welded in place between the upper and lower rings 24, 26 by circumferentially extending welds 36, 38. The rings 24, 26 are, in turn, welded to the torus 18 by means of circumferential welds 40, 42. The nozzle box 12 is supported within the turbine inner shell 44 by a plurality of lugs 46 (one shown) welded to the outside of the torus 18 and bridge ring assembly 20, in an area radially adjacent the steampath assembly 22. The nozzle box 12 is also keyed to the inner shell 44 at 48.
It has not been the practice with the above construction to repair the steampath assembly 22 in situ. Rather, at scheduled maintenance periods, the entire nozzle box 12 would be removed and appropriate repairs made to the partition elements 34 and other components as necessary.
In a later development, as illustrated in FIG. 2 (common reference numerals, but with the prefix "1" added, are used to designate common components), the steampath assembly 122 was manufactured as a one-piece unit including an upper or radially outer band 132 and a lower or radially inner band 130, with integral partitions 134 located therebetween. This new one-piece steampath 122 has superior erosion resistance as compared to the earlier, multi-piece assembly 22 and, in fact, generally does not require repair and/or replacement. Here again, it is to be understood that the one-piece steam path is a 180.degree. segment, and that a pair of such segments are necessary to form a 360.degree. steampath in the final turbine assembly.
This invention relates to the solution to a problem experienced by customers owning earlier turbines having the nozzle box construction shown in FIG. 1, but desirous of having the older multi-piece steampath assembly 22 replaced with the new, one-piece steampath 122 shown in FIG. 2. The prior process for steampath replacement started with removal of the box support lugs 46 to gain access to the original steampath assembly 22. The steampath assembly 22 was then machined off and a transverse weld prep was machined on the inner and outer walls of the box 12. After several intermediate steps, the new erosion-resistant steampath 122 was welded to the bridge ring assembly 20 (or torus 18 if the box was bridgeless) so as to produce the configuration shown in FIG. 2. After the welds were stress-relieved then X-rayed, new lugs (similar to lugs 46) were welded to the box 12. The cycle time to remove, reweld new lugs, stress-relieve the lug weld, and X-ray the lug weld was approximately two weeks.